Method of electrostatic recording on a thermoplastic recording element

ABSTRACT

A METHOD OF RECORDING INFORMATION ON A THERMOPLASTIC RECORDING ELEMENT COMPRISES (1) DISPOSING THE THERMOPLASTIC SURFACE OF THE RECORDING ELEMENT IN A SOLUTION OF CHARGED PARTICLES, AND (2) ELECTRODEPOSITING, AS BY ELECTROPLATING OR ELECTROPHORESIS, THE PARTICLES FROM THE SOLUTION ONTO ONLY A SELECTED AREA OF THE SURFACE, REPRESENTATIVE OF THE INFORMATION, IN A QUANTITY CORRESPONDING TO AN AVERAGE THICKNESS OF AT LEAST 2X10**-4 MONOLAYER OF DEPOSITED PARTICLES, BUT SUBSTANTIALLY INVISIBLE TO THE NAKED EYE. THE   RECORDING ELEMENT WITH ELECTRODEPOSITED INFORMATION THEREON MAY BE STORED AS LONG AS DESIRED, AND THE INFORMATION CAN BE MADE VISIBLE, WHEN DESIRED, BY (3) CHARGING THE SURFACE WITH A UNIFORM ELECTROSTATIC CHARGE, AND (4) HEATING THE RECORDING ELEMENT TO A TEMPERATURE AT WHICH THE THERMOPLASTIC MATERIAL DEFORMS AND PROVIDES A VISIBLE CRAZED IMAGE OF THE INFORMATION.

April 13,l 1971 F. H. NICOLL KETHQD OF ELECTROSIITIC RECORDING 0N ATHERMOPLASTIC RECORDING ELEMENT med nay s1'. 196s 2 Sheets-Sheet 1AYMINIY Apnl'13, 1971 F. H. yNICOLI.

METHOD OF ELECTROSTATIC RECORDING ON A THERMOPLASTIC RECORDING ELEMENTFiled May .'51, 1968 2 sheets-sheet United States Patent,v

3,574,613 METHOD F ELECTROSTATIC RECORDING 0N A THERMOPLASTIC RECORDINGELEMENT Frederick H. Nicoll, Princeton, NJ., assignor to RCA CorporationFiled May 31, 1968, Ser. No. 733,496 Int. Cl. G03g 13/22 U.S. Cl. 96-1.15 Claims ABSTRACT 0F THE DISCLOSURE A method of recording information ona thermoplastic recording element comprises (l) disposing thethermoplastic surface of the recording element in a solution of chargedparticles, and (2) electrodepositing, as by electroplating orelectrophoresis, the particles from the solution onto only a selectedarea of the surface, representative of the information, in a quantitycorresponding to an average thickness of at least 2 10-1 monolayer ofdeposited particles, but substantially invisible to the naked eye. Therecording element with the electrodeposited information thereon may bestored as long as desired, and the information can be made visible, whendesired, by (3) charging the surface with a uniform electrostaticcharge, and (4) heating the recording element to a temperature at whichthe thermoplastic material deforms and provides a visible crazed imageof the information.

BACKGROUND OF INVENTION This invention relates generally to the art ofelectrostatic printing, and more particularly to an improved method ofrecording information on a thermoplastic recording element. The improvedmethod is particularly useful for recording information economically ona recording element at a relatively high speed in a manner whereby therecording element either may be stored for relatively long periods oftime Without development or may be developed at will.

It has been proposed to record information on a thermoplastic recordelement by producing a latent electrostatic image on a layer ofthermoplastic material coated with a thin lm of substantiallyincompatible inelastic material. The latent electrostatic image isdeveloped by heating the thermoplastic layer to soften it and to causethe inelastic thin film thereon to craze, whereby to produce a lightscattering image of the recorded information. Such a method is describedin U.S. Pat. No. 3,317,- 315, issued to F. H. Nicoll and H. F. Ogawa, onMay 2, 1967, for Electrostatic Printing Method and IElement, assigned tothe same assignee as the instant invention. While such a prior-artmethod of electrostatic recording is suitable for many purposes, therecording element cannot be stored for long periods of time withoutdeveloping the latent electrostatic image thereon. Unless the latentelectrostatic image is developed by heating the thermoplastic materialof the recording element to provide a crazed image of the informationwithin a relatively short time, the electrostatic charge will decayrapidly and eventually disappear.

In accordance with the improved method of the present invention,information can be recorded on a thermoplastic recording element, atrelatively high speeds, and the recording element may be stored for anyperiod of time and developed only when desired.

solution of charged particles, onto a selected area of the surface of athermoplastic layer of a recording element.

ICC

The particles are deposited in a quantity corresponding to an averagethickness of at least 2X lil-4 monolayer of deposited particles, butsubstantially invisible to the naked eye. The information may berendered visible subsequently by charging the surface of the recordingelement and heating the thermoplastic layer to a temperature at which itdeforms to provide a visible crazed image at the site of the depositedparticles.

In one embodiment of the improved method, the electrodeposition isprovided by a current of charged particles that is controlled byphotoconductive means. In another embodiment of the improved method, theelectrodeposition of the charged particles is controlled by the size ofthe electrodes and/or modulating pulses applied to the electrodes.

By the terms electrodepositing or electrodeposition, as used herein, ismeant the electrical migration of charged particles (ions or chargedmolecules) in an electric current between electrodes connected to anunidirectional voltage source, as in electroplating or electrophoresis,and depositing the particles adjacent one of the electrodes.

The improved method has the advantage of accepting information on athermoplastic recording element at relatively high speeds in the form ofelectrodeposited material so that the recording element may be storedfor long periods until such time as it is desired to render theinformation visible, as by subsequently electrostatically charging andheating the recording element to form a crazed image of the information.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a llow chart of operationsin the improved method;

FIG. 2 is a cross-sectional view of apparatus, partly schematic, forelectrodepositing charged particles, representative of information, on arecording element having a layer of thermoplastic material, inaccordance with one embodiment of the improved method;

FIG. 3 is a cross-sectional View of apparatus, partly schematic, forelectrodepositing charged particles, representative of information, on arecording element having a layer of thermoplastic material that is alsophotoconductive, in accordance with another embodiment of the improvedmethod;

FIG. 4 is a cross-sectional view of apparatus, partly schematic, forrecording information on a thermoplastic recording element byelectrolytically removing portions of material from a thin film ofparticles on a thermoplastic layer of a recording element, in accordancewith still another embodiment of the improved method; and

FIG. 5 is a perspective view, in cross-section, and partly schematic, ofapparatus for recording information continuously on a web of athermoplastic recording element, in accordance with `a furtherembodiment of the improved method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of thedrawings, there is shown a flow chart comprising the operations of theimproved method for recording information on a recording element havinga thermoplastic layer. In the improved method, the operations areperformed upon a recording element that has a thermoplastic layer,either photoconductive or non-photoconductive of the types described inthe aforementioned US. Pat. No. 3,317,315, or in U.S. Pat. No.3,291,600, issued to F. I-I. Nicoll on Dec. 13, 1966, forElectrophotographic Recording Element and Method of Making.

The method of recording information on the recording element comprisesdisposing the surface of the recording 3 element in a solutioncontaining charged particles, i.e., ions or charged molecules. Thecharged particles are electrodeposited from the solution onto one ormore selected areas of the lsurface of the recording element by theprocess of either electroplating or electrophoresis, depending upon thekind of charged particle employed.

The electrodeposition of the particles on only selected areas of thesurface of a recording element, representative of the information, iscontrolled by varying the resistance to the ow (current) of theparticles in the solution between oppositely polarized electrodes. Thiscontrol may be provided either by photoconductive modulation to vary theelectrical conductivity of the current of particles to selected areas ofthe recording element or by employing electrodes of different sizes inthe electroplating or electrophoretic process.

The deposition of particles on a selected area of the surface of therecording element is in a quantity corresponding to an average thicknessof at least 2 104 monolayer of deposited particles, but substantiallyinvisible to the naked eye. This thickness corresponds to 2 1011ions/cm.2 or 3x10-8 coulombs/cm-2. Such minute amounts of depositedparticles can be accomplished easily in a matter of a few microseconds.Thus, if the resistance through l cm.2 of the thermoplastic layer is 102ohms, for example, and the electrodeposition voltage is volts, 10*1amperes of current can flow and 2X1()11 ions/cm?, the minimum detectabledeposit, can be deposited in 3 X 10iFl seconds.

After the information is recorded on the recording element, therecording element may be stored until such time as it is desirable todevelop the recorded information to visualize it. The information isvisualized by electrostatically charging the surface of the recordelement with a uniform electrostatic charge in a manner well known inthe electrophotographic art, heating the recording element to soften thethermoplastic layer and to form a crazed image at the site of theelectrodeposition of particles, and `subsequently cooling the recordingelement to freeze the crazed image.

Referring now to FIG. 2 of the drawings, there is shown apparatus 10 forrecording information on a recording element 12, having a transparentthermoplastic layer 14 of heat deformable material, in accordance withthe improved method. The recording element 12 comprises a substrate 16,preferably of a transparent material, such as glass or Mylar plasticmaterial. An electrode 18, in the form of a conductive layer of atransparent electrically conductive material, such as tin oxide, iscoated on the substrate 16, and the thermoplastic layer 14 is coated onthe electrode 18. An electrode 20 is disposed adjacent and parallel to,but slightly spaced from, the surface 22 of the recording element 12.The electrode 20 comprises a conductive electrode layer 24 which may beeither a conductive coating of tin oxide on a `transparent substrate, ora sheet of electrically conductive material. A photoconductive layer 28of a photoconductor such as cadmium sulfide or cadmium selenide, forexample, is coated on the electrode layer 24. The electrode 20 may alsoconsist of a single sheet of a photoconductor, such as a sheet ofselenium, for example.

The recording element 12 and the electrode 20 are i disposed in asolution 26 within a transparent container 30, preferably glass, by anysuitable means (not shown). The surface 22 of the recording element 12is substantially parallel to the electrode 20 and is as close to it aspossible without contacting it. A source of unidirectional voltage V isconnected with its positive terminal to the electrode 20 and itsnegative terminal connected to the electrode 18 of the recording element12.

The solution 2'6 may comprise either an electrolyte, such as coppersulfate, for example, containing sulfate anions and copper cations, or acolloidal solution of electrically charged particles, such as a liquiddeveloper of the type used in electrophotography. In each case, the

charged particles will provide a current of particles under theinfluence of a suitable voltage V `so that particles of one charge willmove to an electrode of the opposite polarity.

Means are provided to control the current of charged particles betweenthe electrodes 18 and 20 in the apparatus 10 in response to theinformation to be recorded. To this end, a lens system, represented by alens 32, is disposed so as to project a light ima-ge, as from aphotographic transparency 34, having light-transmitting portionsrepresentative of the information to be recorded, onto thephotoconductive layer 28. A similar light image can also be projectedonto the photoconductive layer 28 by a lens system 32a, from an oppositeside of the electrode 20, if the electrode 2()` has either a transparentsubstrate or is entirely photoconductive.

The improved method of recording information on the surface 22 of therecording element 12 with the apparatus 10 of FIG. 2 will now bedescribed. If the solution 26 is an electrolyte, such as anelectroplating solution of copper sulfate, for example, and if theresistance through l cm.2 of the thermoplastic layer 14 is about 102ohms, and the voltage V is such as to cause 10'"1 amperes of current toflow, information can be recorded on the surface 22 in about 3 X10FIseconds. To do this, a light image of information, such as alight-transmitting portion of the transparency 34, is projected onto thephotoconductive layer 28 while a voltage of about 10 volts, for example,is applied between the electrodes 18 and 20. Consequently, a current ofcopper ions ows through the solution 26 and copper is deposited on thesurface 22 of the recording element 12. Since the resistance to the flowof the plating current is greatly reduced by the decreased resistivityproduced between the light image projected onto the photoconductivelayer 2S of the electrode 20 and the electrode 18, the copperelectrodeposited onto the surface 22` of the recording element 12 isrepresentative of the information to be recorded.

The `quantity of particles, that is, copper, electrodeposited from thesolution 26 onto the surface 22 under the conditions described,corresponds to an average thickness of at least 2X10"4 monolayer ofcopper. This quantity is substantially invisible to the naked eye. While2x10*4 monolayer of deposited material is a lower limit of the quantityof particles that must be deposited on the surface 22 of the recordingelement 12 to provide a subsequent crazed image, the upper limit ofelectrodeposition may be substantially more. It is desirable, however,to deposit the material in as short a time as possible, but the amountof material deposited need not be visible to the naked eye.

The solution 26 can also be a colloidal solution of charged particles,such as electrically charged carbon particles in electrophotographicdeveloping solutions well known in the electrophotographic art. When thesolution 26 is colloidal, and the particles are positively charged, forexample, the particles can be electrodeposited on the surface 22 of therecording element 12 with a suitable voltage V by the process ofelectrophoresis substantially as described for the electroplating ofcopper.

After recording the information by the electrodeposition of theparticles on the surface 22 of the recording element 12, the recordingelement 12 can be removed from the solution 26 and stored until suchtime as it is desired to visualize the stored information. The storedinformation may be visualized by electrostatically charging `the surface22 with a uniform electrostatic charge and by heating the chargedrecording element 12, as by placing the recording element 12 on a hotplate or by directing a stream of hot air onto the surface 22, until thethermoplastic layer 14 deforms and produces a crazed, light-scatteringimage at the site where the particles were deposited. After crazing hasoccurred, the heat is removed from the recording element 12, and thecrazed image of the information remains frozen in the cooled recordingelement 12.

Referring now to FIG. 3, there is shown apparatus a somewhat similar tothat of the apparatus 10 in FIG. 2, similar parts having the samereference number and modified parts being represented by a referencecharacter consisting of both the original reference number and an addedletter. The apparatus 10a is shown comprising a recording element 12a,having a substrate 16, and electrode 18, and a photoconductivethermoplastic layer 14a. Photoconductive thermoplastic layers, such asare included in the recording element 12a, are described in theaforementioned patents.

An electrode a comprises an electrically conductive metal disposedsubstantially parallel to, and slightly spaced from, the surface 22a ofthe recording element 12. The recording element 12a and the electrode20a of the apparatus 10a in FIG. 3 are disposed in a solution 26 withina container 30, substantially as described for the recording element 12and the electrode 20 of the apparatus 10 in FIG. 2.

Information, represented by one or more light-transmitting portions ofthe transparency 34 in FIG. 3 is recorded on the surface 22a. of therecording element 12a as follows. In accordance with the improvedmethod, a light image of the information is focused onto thephotoconductive thermoplastic layer 14a of the recording element 12a bythe lens system 32. Since the photoconductive thermoplastic layer 14ahas a conductivity that varies directly with light intensity, itsconductivity is markedly increased at the site of the projected lightimage thereon. Consequently, when a suitable voltage V (about 10 volts)is applied between the electrodes 18 and 20a, with the polarity shown inFIG. 3, a current of positively charged particles is caused to depositat the site of the projected light image on the surface 22a.

The recording element 12a may now be removed from the solution 26,rinsed, dried, and stored until such time as it is desired to visualizethe information thereon. A crazed image of the information is producedin the same manne-1' described for producing such an image on therecording element 12 in FIG. 2. Thus, a visible crazed image can beproduced by uniformly charging the surface 22a of the recording element12a, as from a corona discharge device in a manner well known in theelectrophotographic art, and subsequently heating the recording element12a until the photoconductive thermoplastic layer 14a deforms andproduces the crazed image. Upon cooling the recording element 12a, thecrazed image remains fixed on the surface 22a and may be viewed eitherby the naked eye or through an appropriate optical projector.

Referring now to FIG. 4, there is shown apparatus 10b for producinginformation on the surface 22b of a recording element 12b. The recordingelement 12b has a substrate 16, an electrode 18 and a photoconductivethermoplastic layer 14a similar to those described for the recordingelement 12a in FIG. 3. In addition, the recording element 12b has auniform film of particles, such as copper, for example, deposited on thesurface of the photoconductive thermoplastic layer 14a in a quantitycorresponding to a thickness of at least 2 10f4 monolayer, butsubstantially invisible to the naked eye. An electrode 20a is disposedparallel to, and slightly spaced from, the surface 22b of the recordingelement 12b by any suitable means (not shown) Within a container 30filled with a solution 26.

The voltage V (about 10 volts) applied between the electrodes 18 and 20ain the apparatus 10b is of a reverse polarity to the voltage V appliedto the apparatus 10a in FIG. 3. Thus, in FIG. 4, information is recordedon the surface 22b of the recording element 12b by removing particlesfrom a selected area of the film 36, leaving the remainder of theunremoved film 36 to define that information. Hence the information isrecorded on the recording element 12b in the form of a negativerecording, the

information recorded on the recording elements 12 and 12a in FIGS. 2 and3, respectively, being considered in the form of a positive recording.

Information is recorded on the surface 22b of the recording element 12bin FIG. 4 in the following manner. In accordance with the improvedmethod, a unidirectional voltage (about l0 volts) is applied with itspositive terminal connected to the electrode 18 and its negativeterminal connected to the electrode 20a. A light image of theinformation on the transparency 34, representative of the information tobe recorded, is projected onto the photoconductive thermoplastic layer14a by the lens system 32. The decreased resistivity, that is, theincreased conductivity, at the site of the projected information causesparticles to be removed from the conductive film 36, leaving particlesof the film 36 on the remainder of the photoconductive thermoplasticlayer 14a in lthe dark portions. The recording element 12b can now bestored for as long as desired, and the information may be visualized ascrazed images, when desired, by the aforementioned operations ofuniformly charging the surface 22b, heat deforming the photoconductivethermoplastic layer 14a, and cooling the recording element 12b. Theinformation on the surface 22b is represented as a clear surface,defined by a crazed background resulting from the portion of the film 36that has not been removed in the recording process.

The improved method is also applicable for continuously recordinginformation on the surface of a moving web of a recording element.Apparatus 40, shown in FIG. 5, illustrates means for carrying out thisembodiment of the improved method. A recording element 42 comprises athermoplastic layer 14 on an electrically conductive backing 44, such asa metal foil or a relatively conductive paper, for example. A web of therecording element 42 is disposed for movement around a metal roller 46,in a direction indicated by the arrows 48 and 50', by any suitable means(not shown). The roller 46 is connected to a source of referencepotential, such as ground, and functions as an electrode in a manner tobe hereinafter described. The roller 46 is disposed in a solution 26 ofcharged particles within a container 52.

A plurality of parallel electrodes 53, 54, 55, 56 and 57 are disposed sothat their tip ends are adjacent to, and very slightly spaced from,different areas of the surface 58 of the thermoplastic layer 14 of therecording element 42. The electrodes 53-57 are connected to a source 60of unidirectional voltage through a plurality of relay operated switches63-67, respectively, in an electrode selection matrix 68. The electrodeselection matrix 68 is also connected to a source of information pulses,as through relay switches 63-6'7, whereby the source 60 of Voltage maybe applied selectively to one or more of the electrodes 53-57 inaccordance with information pulses supplied to the matrix 68. Apparatusof the type for providing a source of information pulses to an electrodeselection means useful in the improved method is described in detail inU.S. Pat. No. 3,060,432, issued to F. A. Schwertz, on Oct. 23, 1962, forElectrostatic Recording of Information.

The negative terminal of the voltage source 60 is connected to thesource of reference potential, ground, and its positive terminal isconnected to the electrode matrix 68 for applying a selected voltagepulse to an appropriate one or more of the electrodes 53-57, inaccordance with information pulses.

In operation, the web of the recording element 42 is moved in contactwith, and around, the roller 46 within the solution 26 of chargedparticles. In response to the information pulses, voltage pulses areapplied between one or more of the electrodes 53-57 and the roller 46,and the charged particles of the solution 26 are electrodeposited by theresulting current. Particles from the solution 26 are deposited onto thesurface 58 of the recording element `42 substantially adjacent the tipends of the electrodes 53-57 because the electrical resistance to thecurrent flow of the charged particles is least in the shortest distancesbetween the tip ends of the electrodes 53-57 and the (electrode) roller46.

The recorded information, that is, the electrodeposited particles on thesurface 58 of the recording element 42, can be stored on the recordingelement 142 for as long a period as desired, and the information may bevisualized, when desired, by uniformly charging the surface 58 with anelectrostatic charge, and heat deforming the thermoplastic layer 14until crazing occurs at the sites of the recorded information. Therecorded information appears .as light-scattering, crazed images whichcan be visualized with the naked eye.

Thus, there has been described an improved method of recordinginformation on a recordingy member having a thermoplastic layer. Whilethe information on the recording member has been defined by particles,either deposited onto the thermoplastic layer or removed therefrom, itis understood that the quantity of the material deposited or removed isa function of the current carried by the charged particles, which inturn, is a function of the intensity of the light of the informationimage and/or the amplitude of the information pulses. Consequently, theamount of deposited material can be varied by modulating the intensityof the light information and/ or the amplitude of the informationpulses. Hence, crazed images of varying tone are produced by informationcontaining modulated light images or modulated information pulses.

What is claimed is:

1. A method of recording information on the surface of a recordingelement comprising a layer of deformable thermoplastic material, saidmethod comprising disposing said surface in a solution comprisingelectrically charged particles,

electrodepositing on a selected area of said surface,

representative of said information, particles from said solution in aquantity corresponding to an average thickness of at least 2X104monolayer of deposited particles, but substantially invisible to thenaked eye,

removing said surface from said solution,

charging said surface with a uniform electrostatic charge, and

heating said recording element to a temperature at which saidthermoplastic material deforms, whereby to provide a visible crazedimage at the site of said deposited particles on said surface.

2. A method of recording as described in claim 1, wherein said layer ofthermoplastic material is disposed on one electrode, and

said electrodepositing on said surface comprises disposing anotherelectrode in said solution adjacent said layer of thermoplasticmaterial, applying a Voltage between said electrodes to provide Iacurrent of said charged particles through said solution, and selective'-ly controlling the flow of the resulting current through said solutionto electrodeposit said particles from said solution only in saidselected area of said surface.

3. A method of recording as described in claim 2, wherein said llayer ofthermoplastic material is photoconductive an said controlling saidcurrent comprises projecting a light image of said information onto saidlayer of photoconductive thermoplastic material, whereby to vary theresistance selectively between portions of said electrodes and toelectrodeposit said particles on said selected area of said surface. 4.A method of recording as described in claim 2, wherein at least one ofsaid electrodes is transparent and comprises a photoconductive layerthereon, and

said controlling said current comprises applying pulses light image ofsaid information onto said photoconductive layer, whereby to vary theresistance selectively between said electrodes and to electrodepositsaid particles selectively on said selected area of said surface. 5. Amethod of recording as described in claim 2, wherein said otherelectrode comprises a plurality of smaller electrodes, each smaller thansaid one electrode, and

said controlling said current comprises applying pulses representativeof said information between said one electrode and selected ones of saidsmaller electrodes,

whereby to cause a migration of said particles only to said selectedarea of said surface, representative of said information.

References Cited UNITED STATES PATENTS 2,758,525 8/ 1956 Moncrieff-Yates9S-1.3 3,291,600 12/ 1966 Nicoll 96-1 3,284,224 11/ 1966 Lehmann 117-1753,291,600 12/1966 Nicoll 96-1 3,317,315 5/ 1967 Nicoll et al. 96-1.13,384,565 5/ 1968 Tulogin et al 204-181 3,404,033 10/ 1968 Laendle117-217 3,475,170 10/1969 Nicoll 96-1.1

GEORGE F. LESMES, Primary Examiner J. C. COOPER I II, Assistant ExaminerU.S. Cl. X.R.

